1. Field of the Invention
The present invention relates to an automotive alternator having a Lundell-type rotor for mounting on an automobile engine, and in particular relates to the construction of a rotor for an automotive alternator which suppresses higher-order numerical component wind noise resulting from rotation of the rotor.
2. Description of the Related Art
FIG. 10 is a cross-section showing the construction of a conventional automotive alternator, and FIG. 11 is a perspective view of the rotor of the automotive alternator in FIG. 10.
A conventional automotive alternator comprises a Lundell-type rotor 7 mounted so as to be freely rotatable by means of a shaft 6 within a case 3 comprising a front bracket 1 and a rear bracket 2 which are made of aluminum, and a stator 8 secured to the inner wall of the case 3 so as to cover the outer periphery of the rotor 7.
The shaft 6 is rotatably supported by the front bracket 1 and the rear bracket 2. A pulley 4 is secured to one end of the shaft 6 to enable a rotational torque from an engine to be transmitted to the shaft 6 by means of a belt (not shown).
Slip rings 9 for supplying an electric current to the rotor 7 are secured to the other end of the shaft 6, and a pair of brushes 10 are housed in brush holders 11 disposed within the case 3 so as to slide in contact with the slip rings 9. A regulator 18 for regulating the output voltage of the stator 8 is adhered to a heat sink 17 fitted on the brush holders 11. A rectifier 12 electrically connected to the stator 8 for rectifying an alternating current generated in the coil of the stator 8 to a direct current is mounted within the case 3.
The rotor 7 comprises a rotor coil 13 which generates a magnetic flux when an electric current flows therein, and a pair of pole cores 20, 21 disposed so as to cover the rotor coil 13 in which magnetic poles are formed by the magnetic flux generated by the rotor coil 13. The pair of pole cores 20, 21 are made of iron, each has a plurality of trapezoidal claw-shaped magnetic poles 22, 23 projecting from an outer circumferential edge thereof at an equiangular pitch circumferentially, and the pole cores 20, 21 are secured to the shaft 6 facing each other so that the claw-shaped magnetic poles 22, 23 intermesh. In addition, fans 5 are secured to both ends of the rotor 7 in the axial direction.
The stator 8 comprises a stator core 15, and a stator coil 16 composed of wires wound around the stator core 15 in which an alternating current is generated by alternating the magnetic flux of the rotor 7 owing to the rotation of the rotor 7.
In a conventional automotive alternator constructed in this manner, a current is supplied from a battery (not shown) by means of the brushes 10 and the slip rings 9 to the rotor coil 13, and a magnetic flux is generated. The claw-shaped magnetic poles 22 of one pole core 20 are magnetized to N polarities by the magnetic flux, and the claw-shaped magnetic poles 23 of the other pole core 21 are magnetized to S polarities. On the other hand, the rotational torque of the engine is transmitted to the shaft 6 by means of the belt and the pulley 4 and the rotor 7 is rotated. Thus, a rotating magnetic field is imparted to the stator coil 16 and an electromotive force is generated in the stator coil 16. This alternating. electromotive force is rectified to a direct current by means of the rectifier 12, its voltage is regulated by the regulator 18, and the battery is recharged.
Now, in a general automotive alternator, because the inner circumferential surface of the stator coil 16 is irregular and the space between the stator coil 16 and the claw-shaped magnetic poles 22, 23 is narrow, wind chopping noises are generated as the rotor 7 rotates. Thus, in the above conventional automotive alternator, the wind chopping noises generated by the portions of the stator coil 16 and claw-shaped magnetic poles 22, 23 facing each other are suppressed by forming the shoulder portions of claw-shaped magnetic poles 22, 23 into a bevel or a curve, as shown in FIGS. 12 and 13. The suppression of the wind chopping noises generated by the portions of the stator coil 16 and claw-shaped magnetic poles 22, 23 facing each other by forming the shoulder portions of claw-shaped magnetic poles 22, 23 into a bevel or a curve in this manner has already been proposed in Japanese Patent No. 2617002 and Japanese Patent Laid-Open No. HEI 7-222415.
In a conventional automotive alternator constructed in this manner, the generation of wind chopping noises is suppressed by forming the shoulder portions of claw-shaped magnetic poles 22, 23 into a bevel or a curve, but the suppression of wind chopping noises is insufficient.
However, as a result of having diligently searched for the source of the wind chopping noises, the inventors have discovered that, besides the portions of the stator coil 16 and claw-shaped magnetic poles 22, 23 facing each other, unpleasant higher-order numerical component wind chopping noises are also generated by the diametrically outermost leading-edge face portions (indicated by P in FIG. 11) positioned on the leading edges in the direction of rotation of the rotor 7 at both axial ends of the diametrically outermost portions of the claw-shaped magnetic poles 22, 23.
In the conventional automotive alternator, no consideration has been given to the wind chopping noises generated by the diametrically outermost leading-edge face portions of the claw-shaped magnetic poles 22, 23, and consequently one problem has been that the wind chopping noises could not be sufficiently suppressed.
Another problem has been that when more material than necessary is removed from the claw-shaped magnetic poles 22, 23 to form the shoulder portions of claw-shaped magnetic poles 22, 23 into the bevel or the curve, the magnetic resistance in that portion increases and the magnetic flux generated by flowing an electric current in the rotor coil 13 during power generation decreases, reducing output.